(1) Field of the Invention
The present invention relates to an underwater projectile that incorporates a ventilation gas jet emitting from a tip of the underwater projectile and a propellant gas jet emitting from a rear of the projectile. The gas jets are produced in a combustion chamber in which the forward-directed ventilation gas jet produces a virtual cavitator to form a gas bubble around the projectile body and the comparatively larger rear-directed propellant gas jet nozzle acts a propellant for the projectile and allows the gas bubble to act as a supercavitator by the moving direction of the projectile.
(2) Description of the Prior Art
Presently, research is ongoing for the use of underwater gun systems as anti-mine and anti-torpedo devices. An underwater gun system is typically composed of a magazine of underwater projectiles, an underwater gun, a ship-mounted turret, a targeting system, and a combat system.
Specifically, the targeting system identifies and localizes an undersea target. The combat system provides the control commands to direct the ship-mounted turret to point the underwater gun towards the undersea target. The underwater gun shoots the underwater projectiles in which the underwater gun is designed for neutralization of undersea targets at relatively long range (200 m for example).
Projectiles fired from underwater guns can effectively travel long distances by making use of supercavitation. A typical supercavitating projectile 10 is depicted in FIG. 1. Supercavitation occurs when the projectile 10 travels through water at very high speeds and a vaporous cavity 12 forms at a tip 14 of the projectile. With proper design, the vaporous cavity 12 can envelop an entire projectile. Because the projectile 10 is not in contact with the water (excluding at the tip 14 and occasional collisions with the cavity wall, “tail slap”), the viscous drag on the projectile is significantly reduced over a fully wetted operation.
Current projectiles lack propulsion in that the projectiles are instead launched from a gun at high speeds (of the order of 1000 meters/second). The projectiles decelerate as they travel downrange toward their targets, striking their target at velocities typically of 500 meters/second.
It is possible to reduce the velocity needed for launch if the projectile is provided with an on-board propulsion system and/or a drag reduction system. If a simple propulsion system is provided, the gun can launch the projectiles at their cruise velocity (desired impact velocity) and the propulsion system can maintain and carry the projectile to its target at approximately the cruise velocity.
A related issue in projectile operation is the problem of speed and depth dependency of a generated cavity. At launch, a cavity is formed, the size of which is a function of the projectile speed and the cavitator size. As the projectile begins to travel down-range, the projectile begins to slow down due to the drag generated at the tip of the projectile and the cavity, that the projectile generates, shrinks. The cavity continues to shrink as the projectile decelerates until the cavity can no longer envelop the entire projectile.
Pressure also influences the size of the cavity. The size of the cavity is inversely proportional to the ambient pressure. Consequently, projectiles cannot travel as far when deep beneath the ocean surface as the projectiles can travel at very shallow depths.
The high ambient pressure of deep ocean depths can be compensated through the injection of gas into the cavity. If gas is forced into the normally vaporous cavity, the internal pressure of the cavity increases and the cavity grows.
It has been demonstrated that forward-directed jets from moving vehicles can produce supercavities in a manner similar to a physical cavitator. The jet advances forward of the vehicle to where a moving front is produced. The size and shape of the cavity are related to the diameter of the forward-directed jet and the speed of the advancement of the front.
Empirical relations, can be used to determine the size of a cavity produced by a disc cavitator. The cavity shape is assumed to be elliptical as defined by
                                                                        (                                                      x                    -                                          l                      /                      2                                                                            l                    /                    2                                                  )                            m                        +                                          (                                  r                  R                                )                            n                                =          1                ,                            (        1        )            where x is the distance along the cavity axis, l is the length of the cavity, r is the cavity radius, and R is the maximum cavity radius. The exponents are selected as m=2 and n=2.4. Two other parameters are required to define the cavity shape: λ(σ) and μ(σ, CD). CD is the cavitator drag coefficient based on the cavitator projected area and σ is the cavitation number defined as
                    σ        =                                            P              ∞                        -                          P              c                                                          l              /              2                        ⁢            ρ            ⁢                                                  ⁢                          U              2                                                          (        2        )            where ρ is the fluid density, P∞ is the ambient pressure, Pc is the cavity pressure, and U is the projectile speed. The first parameter, the ratio of the maximum cavity diameter to cavitator tip diameter ratio is given by
                    μ        =                                                            C                D                            ⁡                              (                                  l                  +                  σ                                )                                                    σ              ⁡                              (                                  1                  -                                      0.132                    ⁢                                                                                  ⁢                                          σ                                              1/7                                                                                            )                                                                        (        3        )            The second parameter, the cavity slenderness ratio, 1/2R, is given byλ=1.067σ−0.658−0.52σ0.465  (4)
The drag coefficient of a disc cavitator is assumed equal to 0.814.
An equivalence is assumed between a jet and a disc. A forward jet cavitator of known cross-sectional area will produce a cavity equivalent in size and characteristic to a disc a fraction of the size. The long supercavity can be formed by the jet. The cavity form will be in accordance with the cavity formed by equivalent disk size if the jet section area is S and the disk area is Sc, the correlation Sc=0.205 S will be satisfied. As such, an improvement in a cavitating-type projectile is to provide a projectile which provides propulsion to maintain a cruise velocity of the projectile while also providing a forwarded-directed jet for supercavitation of the projectile during travel.